1. Field of the Invention
The present invention relates to an error compensating device for calibrating a movement accuracy of a movable unit of a numerical control machine tool and successively compensating an error during its movement.
2. Description of the Prior Art
For recent numerical control machine tools, high working precision is intended, and many new technologies have been developed and utilized. The new technologies are based on the concept of a machine tool structure having a high rigidity in order to prevent dynamic movement disturbance from occurring even during a high-speed movement and a highly reproducible movement mechanism is employed so as to always exhibit similar behavior during the repetition of the movement. However, even when such a design countermeasure is taken, the movement error cannot be completely eliminated because of the thermal deformation of the machine tool structure or the like. Therefore, after the completion of the machine tool, a calibrating operation called calibration is usually performed as required at certain time intervals to measure machine movement errors, and the measurement results are stored and held in a numerical control device. In actually operating and controlling the machine tool, these errors are reflected in control information to compensate an operating command so that the actual movement may not cause the errors.
In a usual calibration method, special measuring apparatus such as a laser interferometer, a dial gauge or the like is mounted on the machine tool. However, it is difficult to collectively and automatically measure various movement errors such as roll, pitch, yaw or the like of a movement axis, the degree of movement parallelism, movement perpendicularity or the like and right angles among a plurality of movement axes or other movement errors. Therefore, a setting-up operation exclusive for each error measurement is manually performed, and errors are measured and verified taking a relatively long period of time (at least several hours) in the present situation.
Also, in a precise calibrating operation, when utilizing light measurement as a measuring means, in principle a distance is generally measured based on a light wavelength.
In the above calibration method, the machine tool is occupied during every calibrating operation, the method can not be used during normal working operation, and the machine operation rate is decreased. Therefore, the calibrating operation can not be performed frequently. Also, to prevent the measuring accuracy from being varied by external disturbances such as temperature variation or occurrence of vibration in the measuring environment during the measuring operation, a remarkably stable measuring environment needs to be secured.
Also, even the machine tool developed basing on the above-mentioned new design concept inevitably has an error which can not be eliminated so completely as the error caused by the machine thermal deformation. It is known that this error generally takes irregular and complicated behavior as time elapses. It is also known that this behavior variation gradually takes place for a comparatively long period of time from several ten minutes to several hours. Therefore, when the calibrating operation in the conventional method is carried out at time intervals from several ten minutes to several hours, the machine movement error can be effectively compensated. However, this is not actually possible in view of the secured operation rate of the machine tool.
Further, since the accuracy of measurement based on the light wavelength is easily affected by temperature, air turbulence, humidity or the like, there is a problem that the measurement can not be easily utilized in a general factory environment where machine tools are operating.
On the other hand, because a demand for improvement in product quality is getting severe year by year, development of new technology for solving these shortcomings of the conventional calibrating method is desired.
An object of the present invention is to solve the above-mentioned problems, to detect a spatial movement error of a machine tool changing with elapsed time caused by the thermal deformation of the machine tool, abrasion of a movement mechanism or deterioration of assembly accuracy in the normal operation condition including a working operation without being influenced even by the general factory environment, to control the movement of the machine tool while compensating the machine movement error based on the result of detection, and to operate the machine tool with high accuracy.